The instant invention relates to apparatus for detecting the presence of a magnetic field disturbance. More particularly, the instant invention relates to a magnetic field detector having a pair of spaced-apart field sensors for sensing a variation in the earth's magnetic field caused by the presence of a magnetic anomaly in the vicinity of the magnetic field detector apparatus.
An effective device for measuring magnetic fields and locating magnetic anomalies is the magnetometer. The capability of magnetometers to detect the presence of magnetic anomalies is well known and has found significant use. One magnetometer that has been found to be suitable for detecting the presence of ferromagnetic bodies is the proton magnetometer. This magnetometer is based in principle upon the phenomenon of free nuclear precession.
One instrument in the prior art that utilizes proton magnetometers to detect the presence of ferromagnetic bodies is that described and shown schematically in a book, Physics and Archaeology, authored by M. J. Aitken and published in London in 1961 by Interscience Publishers, Inc. The instrument shown therein utilizes two field sensors separated by a distance and used to measure the average gradient over the distance. The sensors themselves each comprise a bottle of liquid having a coil of wire placed around the outside of the bottle. the two coils are connected in series and are supplied with a polarizing current. Since proton magnetometers operate on the free proton precession phenomenon, if the two bottles are in the same magnetic field strength, the sum of the two signals is merely twice the amplitude of one of the signals and decays to zero. If the bottles experience different fields, however, the frequencies of the signals are different; and the two signals will successively get in and out of phase thereby producing a beat frequency. Because the two sensor signals are being summed, the beat frequency, which is the frequency different between the two component signals, will result.
An output signal comprising the sum of the two individual sensor signals is applied to a high input impedance preamplifier. The output signal from the preamplifier is then applied to the input of an amplifier circuit tuned to pass only signals of a frequency within a particular band. A final amplifier stage is provided which feeds a loudspeaker to produce an audible sound indicative of the presence or absence of a search object detectable by the instrument.
In one of the more particular details of the instrument, the bottle coils, which are connected in series, are pulsed by a common polarizing current through a single relay contact. A timing flip-flop automatically controls the relay in order to alternately connect the bottle coils to a source of polarizing current and to the input of a preamplifier. This arrangement provides "listening" periods between the polarizing periods.
The period of the beat signal indicates the strength of the disturbance, with the length of time required for the best signal to drop to its first zero being the least when the detected object is reached. The loudspeaker is provided to make available a readout indication of the best frequency. In the absence of a magnetic disturbance, a gradually decaying signal of a fixed frequency is heard during each listening period. In the presence of a magnetic disturbance, "bleeps" or beat signals are heard, and the greater the strength of the disturbance--the more beats or bleeps within the listening period that will be heard. As an alternative to an audio readout, it is suggested in the book that the rate of decay of the beat frequency can be timed electronically. Also, the number of oscillations occurring during the listening period or during some portion thereof could be counted to provide a readout of field strength.
Although the instrument described is an effective magnetic anomaly sensor, the design integrates the human ear into the detector loop. To obtain a high degree of accuracy in measuring the strength of a detected magnetic anomaly, it is desirable to measure the period of time between the beginning of a listening period and the first detectable beat signal minimum. The ear's ability to detect a minimum requires a high signal-to-noise ratio, making an accurate determination of small field strengths difficult to obtain with the instrument.
A magnetic field measurement apparatus that utilizes nuclear magnetic resonance in the measurement of a varying magnetic field is that disclosed in U.S. Pat. No. 3,260,926. The apparatus disclosed comprises a magnetometer having an integrating circuit coupled to the output leads of a pair of polarizing coils. A controlled switch is provided which operates in a first position to hold the integrating circuit output at zero and in a second position to apply the voltage induced in the coil windings to the input of the integrating circuit, while simultaneously releasing the output of the integrating circuit.
The signal available from the integrating amplifier circuit is fed to the input terminals of a graphical type recorder. The recorder is disclosed as possibly being an oscilloscope or other form of x-y display. In any event, the resulting visual display produced will show the resonance lines plotted against magnetic field value. Alternatively, the integrating amplifier circuit output voltage could be supplied to an accurate volt meter or digital volt meter. The amplifier output voltage for a given magnetic field intensity will be related by a mathematical relation that involves not only the voltage but the component values of the integrating circuit amplifier and the physical dimensions of the magnetometer probe coil.
Another magnetic field measurement apparatus that utilizes nuclear magnetic resonance type magnetometers is that disclosed in U.S. Pat. No. 3,441,838. The apparatus described therein has a pair of vessels that each contain a material having gyromagnetic properties. Two pairs of coils are used with one coil of each pair being wound about a different one of the vessels. One pair of coils is connected in series and coupled to the input of a linear amplifier. The other pair of coils, also serially connected, are further connected to the output of the amplifier. The output of the amplifier is fed to a frequency meter.